Testing of insulated wire just after fabrication but prior to spooling for delivery usually involves passing the wire, under tension, over an energized roller or sheave in order to induce a fault current through any pinholes or other defects which may exist in the insulation. The wire itself is grounded so that the fault current can be detected by a sensing circuit.
For example, U.S. Pat. No. 3,413,541 entitled APPARATUS FOR DETECTING INSULATION FAULTS IN MAGNET WIRE UTILIZING FIELD EFFECT TRANSISTOR, issued Nov. 26, 1968 to W. A. Swim et al, discloses a test apparatus for detecting faults in the insulation coating of a moving insulated wire. As described by Swim et al, insulated wire such as magnet wire is produced in commercial quantities from bare copper wire by applying enamel to the bare wire and then curing by passing the wire through an enameling oven. To obtain the desired insulation "build," a preselected number of enameling passes are made. Fine magnet wire is built up at speeds of many hundreds of feet per second and many strands of wire may be fabricated in parallel at one time. In the Swim et al apparatus, a relatively high direct current potential is applied by a contact wheel to the surface of the insulation coating of the moving wire. The contact wheel is electrically connected to a voltage divider having a first branch connected to a DC source and a second branch connected to a sensing circuit utilizing a field effect transistor (FET) to detect changes in current through the insulation to ground. The FET is switched into a conducting state in response to a predetermined drop in the current flow in the second branch thereby indicating the occurrence of a fault in the insulation coating of the insulated wire. The current sensing circuit may be connected to a recorder, typically a stripchart recorder, or to another device for recording or counting the faults occurring in the wire insulation. The first branch preferably limits the current supplied to the contact wheel to a magnitude of not more than 25 microamps in order to provide a test apparatus which is essentially ignition proof (to prevent high powered sparks in an explosive atmosphere). Swim et al also disclose that the sensing circuit may be mounted on a plug-in circuit board to facilitate maintenance and replacement of parts. The plug-in circuit board is designed with a field effect transistor sensing circuit which presumably helps provide a high signal-to-noise ratio.
However, the plug-in circuit board disclosed by Swim et al is connected to a separate DC power supply which presumably also supplies any other plug-in circuit boards which may exist for testing other wires in separate channels. This provides an undesirable potential for cross talk between channels, which may set off "sympathetic" faults in channels adjacent to a channel in which a true fault occurs.
Swim et al also discloses use of a strip chart recorder which introduces the difficulty of having to examine many feet of paper which can be time consuming, since production in modern plants is carried out around the clock and therefore testing must be accomplished at frequent periodic intervals involving much expenditure of time in examining stripcharts.
Most of the testers presently in use utilize DC for testing insulation. However, AC testing has been found to be more effective in detecting faults for some applications.
It would therefore be desirable to increase the isolation of separate channel circuit boards and to provide fault signal outputs suitable for digital storage and display in order to eliminate the need to devote substantial amounts of time in examining analog strip chart records. It would also be desirable to provide circuit boards which are capable of operating with either high voltage AC or DC.